As methods for producing common metallic powder, various methods, such as water atomization, gas atomization, a rotating electrode process, and a molten-metal dropping process, have been known. Reduced iron powder and atomized iron powder are versatile as iron powder for powder metallurgy. Water atomization is predominant in the methods for producing atomized iron powder. Iron powder produced by water atomization generally has a particle size of 0.2 mm or less. The iron powder has an irregular shape. Iron powder that is generally spherical is produced by gas atomization. However, the cost for producing powder by that process increases, since a large amount of inert gas is consumed. This is a problem. Shots should have a particle size (a mean particle size) of 0.03 to 4 mm. Thus the range of the particle sizes is wide. Further, the shots should be generally spherical. Thus as a method for producing the shots, the method is required to produce particles of various particle sizes and to produce spherical particles. Under these circumstances, a general method for producing metallic powder with no change cannot be used for producing the shots.
A conventional and well-known method for producing the shots is called a centrifugal method. By that method, a disc and a unit for rotating the disc are provided at the center of a large tank for water. A narrow flow of molten metal is dropped onto the disc by means of a tundish or the like. Molten drops are formed from the molten metal by centrifugal force. The distribution of the particle sizes of the shots made by the centrifugal method can be controlled to some extent by changing the speed of the rotation of the disc (i.e., the speed at the rim of the disc, at which speed the molten drops are ejected). However, that method has a low yield (see U.S. Pat. No. 2,310,590 and Chinese Utility Model No. 2,541,089). This is a problem.
U.S. Pat. No. 2,310,590 relates to a conventional centrifugal method for producing shots made of cast iron. It discloses a method for causing molten drops to be fine by spraying pressurized water onto the flying molten drops in a publicly-known centrifugal method, wherein a disc and a unit for rotating the disc are provided at the center of a large tank for water, wherein a narrow flow of molten metal is dropped onto the disc by means of a runner, and wherein molten drops are formed from the molten metal by centrifugal force. By adding a process of spraying pressurized water, it resolves a problem where coarse particles that are too big are formed. It significantly improves the yield of fine particles that are 0.068 inch (1.73 mm) or less. However, the improvement of the yield is insufficient.
Chinese Utility Model No. 2,541,089 relates to a conventional centrifugal device for producing shots made of steel. The basic configuration of the device, wherein a disc and a unit for rotating the disc are provided at the center of a large tank for water, wherein a narrow flow of molten metal is dropped onto the disc by means of a tundish, and wherein molten drops are formed from the molten metal by centrifugal force, is the same as that in U.S. Pat. No. 2,310,590. It tries to resolve a problem in the increased rate of waste products (the rate of defects) due to the low temperature of the molten metal by the arrangement where the furnace and the tundish are integrally located near the disc. As discussed above, the conventional centrifugal methods and devices cannot sufficiently resolve the problem in the low yield of the products.